In the episode, Kirk and Spock find themselves in New York City, searching for Doctor McCoy in 1930. While lying low, Captain Kirk falls in love with the social workerEdith Keeler. Spock discovers that the timeline has been altered, and that Edith Keeler is the cause. In the original timeline, she was killed in a traffic accident. But in the bad timeline, she goes on to found a pacifist movement, meeting with FDR, and delaying the United States's entrance into World War Two. Because of that, Germany won the war because they developed atomic bombs first, and conquered the world.

So when Spock tells Captain Kirk of this, he ends with, "Jim... Edith Keeler must die."

Kirk wrestles with this, and in the end, throws himself in front of McCoy when he tries to save Keeler from an oncoming car.

The moment was one of the most dramatic in Star Trek history, one of the defining moments of Kirk's character, and is said to be the moment at which many people stopped thinking of Star Trek as just a kids' show.